Are you playing it for the first time after a fresh installation? I seem to remember source games stuttering a whole lot the first time through, until all of their assets had been accessed at least once (mostly an issue when loading speech or starting new chapters with new sets of textures/props). I've never figured out why, and I'm not sure whether it's just an old bug that has since been fixed (I haven't played any source games in a long time), but it was definitely a noticeable issue for me.

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You're a classic example of the inverse ratio between the size of the mouth and the size of the brain.